1. Field of the Invention
This invention relates to electronic music synthesis and in particular is concerned with the creation of musical tones that are harmonically related to the frequency of an actuated keyboard switch.
2. Description of the Prior Art
Electronic musical instruments such as organs, are distinguished from many other musical instruments in that a plurality of tone controls are implemented which produce musical tones at a variety of fundamental pitches. For historical reasons related to the design of wind blown organ pipes, the tone pitches are referred to as footages. The eight-foot pitch, or unison pitch, is one in which if the keyswitch corresponding to A.sub.4 is actuated ("on" switch state) then the fundamental musical frequency is 440 Hertz. In addition to the unison pitch of 8-foot, there commonly are a set of octave pitches. For example, a four foot pitch will sound a tone having a fundamental frequency which is one musical octave higher than the actuated keyswitch. A two-foot pitch tone control will cause a tone to sound which has a fundamental frequency which is two musical octaves above the actuated keyswitch. A 16-foot pitch tone control will cause a tone to sound which has a fundamental frequency which is one musical octave below the actuated keyswitch.
Pipe organs, as well as many electronic keyboard musical instruments, traditionally contain fractional foot stops which are given the generic name of mutations. For example a 22/3-foot pitch stop will cause a tone to sound with a fundamental frequency which is equal to the third harmonic of the actuated keyswitch. A 1 3/5-foot pitch stop will cause a tone to sound with a fundamental frequency which is equal to the fifth harmonic of the actuated keyswitch. It is a relatively easy matter to build mutation tones for a pipe organ in which a separate rank (set) of pipes is associated with each of the stops, or tone controls. One simply tunes the pipes in the mutation rank to sound at the designated true harmonic of the actuated unison keyboard switch.
In most electronic keyboard musical instruments, an economical method of implementing mutation stops is to use a tone control system which has the generic name of unification. For example if a 22/3-foot tone control is actuated and the keyswitch corresponding to the musical note C.sub.4 is actuated, then either by electrical or mechanical linkages, the keyswitch contacts corresponding to the musical note E.sub.6 will be actuated. The linkage arrangement is such that C.sub.4 will not sound and only the unified tone will sound. This system produces a tone whose fundamental is close, although not equal, to the true third harmonic frequency of the actuated keyboard switch. A similar linkage arrangement is used to obtain other mutation pitches such as the 1 3/5-foot pitch which produces an approximation to the true fifth harmonic of the tone corresponding to the actuated keyboard switch.
Accompanying the current trend to implement tone generators using large scale arrays of microelectronic elements has been the design trend to use a number of tone generators which is less than the number of keyswitches on an instrument's keyboard array of keyswitches. Unification methods to obtain mutation pitches can in, these systems, become a fairly costly option. In such system it is necessary to provide an additional separate and independent set of tone generators for each of the required mutation pitches.
In digital tone generators such as those described in U.S. Pat. No. 3,515,792 entitled "Digital Organ;" U.S. Pat. No. 3,809,789 entitled "Computer Organ;" and U.S. Pat. No. 4,085,644 entitled "Polyphonic Tone Synthesizer," higher than unison pitches are obtained by a scheme of harmonic suppression. Thus a 4-foot stop is obtained by using only the even numbered harmonics from the total set of possible harmonics and suppressing all the odd numbered harmonics. A 22/3-foot stop is obtained by using only the harmonic number sequence 3,6,9,12,15 . . . and suppressing all the other harmonics.